1. Field of the Invention
The present invention generally relates to automotive drive systems and, more particularly, to such drive systems of the beam-type straight axle type.
2. Description of the Related Art
Beam-type straight axle drive systems have been and still are commonly used for many different automotive applications. While simple in design, these axle types also tend to be inexpensive and very durable, making them highly desirable on utility vehicles such as pickup trucks and sport utility vehicles (SUVs) which are equipped with 4-wheel drive or, at least, power transmission to the front wheels.
Many types of mounting and/or suspension methods have been used to attach straight axles to vehicles. Such methods include leaf springs, radius arms with coil springs, multi-link systems with coil springs, or a combination of any of the above. Probably the most compliant and well-functioning method of mounting/suspending a straight axle to a vehicle is the multi-link system in combination with coil springs. Vehicles such as the Jeep Cherokee (XJ Model), Jeep Comanche (MJ Model), Jeep Grand Cherokee (ZJ and WJ Models), Jeep Wrangler (TJ Model), and Dodge 4×4 pickup trucks have all used this type of system with reasonable success.
The multi-link system of locating a beam-type straight axle commonly utilizes five different links: two upper suspension arms, two lower suspension arms, and one track bar (see FIG. 1). The combination of these links enables the axle to move and articulate rather freely, while still providing a relatively stable platform for the vehicle to be suspended on.
Additionally, the steering system commonly utilized with this type of suspension system is the Y-type linkage (see FIG. 1). This system consists of a steering drag link running from the steering box pitman arm to the opposite steering knuckle, and a steering tie rod which attaches somewhere along the length of the steering drag link and on the opposite end to the remaining steering knuckle.
There are substantial problems with these types of suspension and steering systems. First, suspension compression results in axle translation, thus causing the vehicle to experience bump-induced yaw. When the vehicle hits a bump in the road, the suspension system will absorb this bump by allowing the axle to travel upward closer to the vehicle frame (see FIG. 2). As this happens, the track bar forces the axle toward the side of the vehicle opposite where the track rod attaches to the vehicle frame or body structure. This translation in axle location actually causes the vehicle body to move in the opposite direction (as the tires and wheels are mounted to the axle, and they will stay planted on the road surface), which is bump-induced yaw. This phenomenon creates a very unstable feel in the vehicle, possibly leading to loss of vehicle control. The problem is only exacerbated when the vehicle experiences increased suspension compression, such as during use in rough terrain.
Another significant problem lies with the Y-type steering linkage. As suspension compresses, the linkage changes its effective length, thus causing a toe-out situation with the tires. When the vehicle hits a bump in the road and the suspension compresses, the steering drag link and steering tie rod change relationship relative to one another (see FIG. 3). They become more parallel to one another during suspension compression, thus increasing the effective distance between the steering knuckles and thereby increasing toe-out on the vehicle (bump-induced toe change). Toe-out tends to make a vehicle “wander” or “hunt” around on the road, or in other words become unstable.
Jeep has somewhat resolved the bump-induced toe change problem on its later-model vehicles (WJ) by attaching the steering tie rod directly to both steering knuckles, rather than one steering knuckle and the steering drag link. However, there still remains an inherent problem with bump-induced steering, or bumpsteer. Referring to FIG. 1, it will be noted that the endpoints of the track rod do not coincide exactly with the endpoints of the steering drag link. As the vehicle hits a bump and the suspension compresses, these two linkage units move through somewhat different arcs. As the axle translates a given distance through a given amount of suspension compression, the steering knuckle translates a different amount. Thus, the vehicle will experience some amount of steering movement due solely to hitting the bump, which is bumpsteer.
Combining these three different issues together can cause a vehicle to handle very poorly during suspension compression, possibly leading to loss of vehicle control. A method of solving these problems is highly desirable for people who own or plan to own any vehicle utilizing these suspension and steering systems.